I created an iPhone smashed to the ground (following a tutorial - cool stuff!).
and after finishing the modeling, texturing, setting up the particles, lighting, and composting in the composting window. i wonted to render a .PNG file (or any other file type), but what do you know - i have no idea how to render out of blenders composting window.

just go back to the Default screen lay-out and in the render panel set your format, output directory etc. and press render. Blender should detect the Compositing setup, if not open the Post Processing tab and select Compositing